The present invention relates to Ball Grid Array (BGA) electronic modules and more particularly to BGA modules for High Frequency applications.
A recent development of technology has introduced the use of Printed Circuit Board (PCB) laminates as substrates for the manufacturing of electronic modules which can be of the Single Chip Module (SCM) type or Multi Chip Module (MCM) type. These modules are provided with a plurality of conductive pads for electrical connection with electronic circuits (such as mother boards, back planes, application boards). The electrical connection is achieved by little spherical portions of solder alloy which give the name of Ball Grid Array (BGA) to this kind of electronic module. Usually such modules use PCB laminates made of organic material. These modules are usually called Plastic Ball Grid Arrays. The definition xe2x80x9cPlasticxe2x80x9d indicates the organic nature of the PCB as opposed to a ceramic substrate. Another example of a BGA module is the Tape BGA (TBGA) which uses a tape of organic material as substrate instead of the laminate.
When the number of input/output leads extending from the electronic devices is very high it is known to build the substrate as a multilayer substrate having multiple layers of conductive material, wherein each layer must be electrically coupled together. This requires creating through holes and/or via vias to interconnect each conductive layer. Through hole drilling and/or via creation is one of the most expensive PCB operations. Each layer of interconnections increases the cost of PCB. In addition, circuits which must travel across one layer, down a through hole and back across another layer are much poorer in electrical performance than circuits which can travel directly across a single layer. For this reason, when possible, a single layer is preferable to a multilayer substrate.
FIG. 1 is an example of a prior art PBGA of the Cavity Down type. In the Cavity Down modules (as opposed to the Chip-Up modules) the active element 107 is attached on the lower side of the module, on the same side of the solder balls 105 and it is positioned in a sort of cavity of the organic substrate 101, which completely surrounds the active element 107. This arrangement presents some advantages with respect to the Chip-Up PBGA module. One of the advantages is the reduced thickness of the resulting package, since the chip is xe2x80x9ccontainedxe2x80x9d in the substrate. Furthermore, these modules provide a better heat dissipation, because the active element is usually attached to a metal stiffener which constitutes the top face of the module and also acts as heat dissipator.
When PBGA packages are used in high-frequency applications they require Electro Magnetic Interference (EMI) shielding to avoid background interference with the product working frequency. The higher the frequency is, the shorter the related wave length. If the wave length is short enough, it can pass through the atomic structures between molecule and molecule and the signal can cross the materials commonly used for the manufacturing of the electronic packages. If this happens, the interferences/disturbances can reach the active circuit on the chip, couple with correct working signals, latching or delatching circuits in a completely arbitrary way, causing the chip functions to be unrecognizable or unusable and, in some cases, even physically damaging the application. In a normal environment, there are several possibilities for random signals with HF/Radio Frequency characteristics, such as electrical spikes, household disturbances, short wave rays (X-Rays) present in the atmosphere and many others. To avoid this major problem, it is necessary to protect the RF application with a kind of box made with a material having a very tight molecular structure, such as a metal, that cannot be crossed by the RF interferences but which reflects them back. This metal box operates as a Faraday cage that protects the application functionality.
It is known to manufacture devices for HF applications using an all metal cavity package to house the electronic circuit. In hybrid microelectronic circuits the substrate (usually ceramic) bearing silicon device is glued or brazed to bottom of the metal package. Then wire bonding interconnections are formed between substrate and package leads. Then the whole module is capped by brazing or welding a cover lid to the open cavity in order to get a one piece all metal package. The EMI shielding is thus obtained by grounding the metal envelope through internal interconnections.
However, this solution involves considerably high costs. Use of low cost organic packages would be very desirable.
EP-A-872888 (and the corresponding U. S. Pat. No. 5,955,789) provides a solution which allows the use of low cost, commonly available, base organic material to manufacture HF devices. EP-A-872888 provides an effective shielding from Electro Magnetic Interference (EMI) by building a sort of complete Faraday Cage, protecting the active device from atmospheric natural electromagnetic radiations of radio frequency, or the resulting disturbances in radio equipment interference.
According to EP-A-872888 a complete shielding of a module using an organic substrate is realized. The organic materials do not constitute a barrier for the HF wave. For this reason a metal fence is created along the sides of the substrate by a combination of solder balls and plated through holes connected together (for example) in a zig-zag way. As schematically shown in the plane view of FIG. 2 plated through holes 201 along the four sides of the organic substrate alternate with solder balls 202 and are connected to each other to constitute a xe2x80x9cfencexe2x80x9d which surrounds the whole module. This fence constitutes a shield which has been proven to be very effective for protecting against high frequency electromagnetic waves. Laboratory tests showed that a shielding like the one described above can be used to protect from a HF radiation of more than 1 GHZ. In a preferred embodiment, these through holes and solder balls are identical to the usual through holes and solder balls used for the manufacture of BGA modules, but have no connection with the active circuits. They are connected to ground for realizing the HF shielding. FIG. 2 schematically shows for example some of the solder balls 205 which realize the signal connections between the active device and the main board to which the module will be mounted.
Referring now to FIG. 3 the complete Faraday Cage realized according to EP-A-872888 is detailed. The active element 301 must be completely surrounded by the Faraday Cage in order to be shielded from the HF electromagnetic waves. The lateral sides of the Cage are constituted as explained above by the plated through holes 201, and by the solder balls 202 connected together. The through holes 201 provide a shielding within the substrate (which is according to the preferred embodiment an organic laminate), while the solder balls ensure a lateral protection between the substrate and the main board (when the module is finally mounted on the board). The ground plane 303 in the main board, properly connected to the solder balls 203, will constitute the lower side of the Faraday Cage, while the upper side, according to the preferred embodiment, will be realized by connecting the top metal plate 305, which usually constitutes the top side of a Cavity Down module, with the through holes 201.
As mentioned above, multilayer substrates have several drawbacks:
they need expensive drilling operations for the creation of through hole;
circuits traveling across the layers have poor electrical performances; and more important
the thickness of the module cannot be reduced as desired.
Examples of applications needing a very thin module are small portable devices as the PCMCIA (Personal Computer Memory Card International Association) where a thickness of 1.2 mm and lower is an objective. Bumped devices do have a thickness between 0.5 to 0.8 mm making such a 1.2 mm objective difficult to reach without a very thin carrier. FR4 carriers thickness cannot be reduced too much otherwise they become very fragile and prone to high level of deformation because of the usage of several materials with different coefficients of thermal expansion, compromising easily the package reliability of the application.
Further product enhancements such as PDA (Portable Digital Assistant) and other portable multimedia applications will embed RF features such the Blutooth standard for wireless local area networking between appliances will drive the overall size of the package to thinner dimensions, and the same for GPS and GSM applications.
It is an object of the present invention to provide a technique which alleviates the above drawbacks.
According to the present invention, we provide a Ball Grid Array (BGA) package including:
a metal stiffener;
a dielectric layer laid on said metal stiffener;
a plurality of circuit traces on the dielectric layer, the traces having one end connectable to an active element mounted on the dielectric layer, and the other end connectable to a solder ball for connecting the active element to a mother board;
peripherally to the plurality of traces, a plurality of metallized photovias, each photovia being connected to the metal stiffener and connectable to a solder ball for connection with the mother board.